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1.
The mineralogy of comet 81P/Wild 2 particles, collected in aerogel by the Stardust mission, has been determined using synchrotron Fe‐K X‐ray absorption spectroscopy with in situ transmission XRD and X‐ray fluorescence, plus complementary microRaman analyses. Our investigation focuses on the terminal grains of eight Stardust tracks: C2112,4,170,0,0; C2045,2,176,0,0; C2045,3,177,0,0; C2045,4,178,0,0; C2065,4,187,0,0; C2098,4,188,0,0; C2119,4,189,0,0; and C2119,5,190,0,0. Three terminal grains have been identified as near pure magnetite Fe3O4. The presence of magnetite shows affinities between the Wild 2 mineral assemblage and carbonaceous chondrites, and probably resulted from hydrothermal alteration of the coexisting FeNi and ferromagnesian silicates in the cometary parent body. In order to further explore this hypothesis, powdered material from a CR2 meteorite (NWA 10256) was shot into the aerogel at 6.1 km s?1, using a light‐gas gun, and keystones were then prepared in the same way as the Stardust keystones. Using similar analysis techniques to the eight Stardust tracks, a CR2 magnetite terminal grain establishes the likelihood of preserving magnetite during capture in silica aerogel.  相似文献   

2.
The bulbous Stardust track #80 (C2092,3,80,0,0) is a huge cavity. Allocations C2092,2,80,46,1 nearest the entry hole and C2092,2,80,47,6 about 0.8 mm beneath the entry hole provide evidence of highly chaotic conditions during capture. They are dominated by nonvesicular low‐Mg silica glass instead of highly vesicular glass found deeper into this track which is consistent with the escape of magnesiosilica vapors generated from the smallest comet grains. The survival of delicate (Mg,Al,Ca)‐bearing silica glass structures is unique to the entry hole. Both allocations show a dearth of surviving comet dust except for a small enstatite, a low‐Ca hypersthene grain, and a Ti‐oxide fragment. Finding scattered TiO2 fragments in the silica glass could support, but not prove, TiO2 grain fragmentation during hypervelocity capture. The here reported dearth in mineral species is in marked contrast to the wealth of surviving silicate and oxide minerals deeper into the bulb. Both allocations show Fe‐Ni‐S nanograins dispersed throughout the low‐Mg silica glass matrix. It is noted that neither comet Halley nor Wild 2 had a CI bulk composition for the smallest grains. Using the analogs of interplanetary dust particles (IDPs) and cluster IDPs it is argued that a CI chondritic composition requires the mixing of nonchondritic components in the appropriate proportions. So far, the fine‐grained Wild 2 dust is biased toward nonchondritic ferromagnesiosilica materials and lacking contributions of nonchondritic components with Mg‐Fe‐Ni‐S[Si‐O] compositions. To be specific, “Where are the GEMS”? The GEMS look‐alike found in this study suggests that evidence of GEMS in comet Wild 2 may still be found in the Stardust glass.  相似文献   

3.
Lunar regolith breccias are temporal archives of magmatic and impact bombardment processes on the Moon. Apollo 16 sample 60016 is an “ancient” feldspathic regolith breccia that was converted from a soil to a rock at ~3.8 Ga. The breccia contains a small (70 × 50 μm) rock fragment composed dominantly of an Fe‐oxide phase with disseminated domains of troilite. Fragments of plagioclase (An95‐97), pyroxene (En74‐75, Fs21‐22,Wo3‐4), and olivine (Fo66‐67) are distributed in and adjacent to the Fe‐oxide. The silicate minerals have lunar compositions that are similar to anorthosites. Mineral chemistry, synchrotron X‐ray absorption near edge spectroscopy (XANES) and X‐ray diffraction (XRD) studies demonstrate that the oxide phase is magnetite with an estimated Fe3+/ΣFe ratio of ~0.45. The presence of magnetite in 60016 indicates that oxygen fugacity during formation was equilibrated at, or above, the Fe‐magnetite or wüstite–magnetite oxygen buffer. This discovery provides direct evidence for oxidized conditions on the Moon. Thermodynamic modeling shows that magnetite could have been formed from oxidization‐driven mineral replacement of Fe‐metal or desulphurisation from Fe‐sulfides (troilite) at low temperatures (<570 °C) in equilibrium with H2O steam/liquid or CO2 gas. Oxidizing conditions may have arisen from vapor transport during degassing of a magmatic source region, or from a hybrid endogenic–exogenic process when gases were released during an impacting asteroid or comet impact.  相似文献   

4.
Abstract– The nakhlites contain small proportions of Cu‐Fe‐Ni sulfide minerals; we have studied these sulfides in Northwest Africa (NWA) 998, Nakhla, Governador Valadares, and NWA 817 with optical microscopy, scanning electron microscope, and electron microprobe. Modal abundances of magmatic sulfides, as estimated by image analysis on thin section, are uniformly low (0.02 to 0.05 ± 0.03 vol%), i.e., a factor 5 lower than in shergottites. Sulfides occur within the glassy mesostasis, as composite two‐phase Fe‐Ti oxide‐sulfide grains, intimately associated with interstitial grains or locally enclosed in postcumulus melt inclusions (e.g., Governador Valadares) in olivine. They exhibit a uniform low‐Ni monoclinic pyrrhotite composition ± chalcopyrite. There is a gradation of sulfide grain sizes and textures across the nakhlites flow(s): droplets in NWA 817; resorbed blebs in Governador Valadares; more massive, true intercumulus blebs in Nakhla and NWA 998. These nakhlites also show evidence for sulfide weathering. Hot desert finds (e.g., NWA 998 and NWA 817) show a few percent fracture‐filling iron (oxy) hydroxides of likely terrestrial origin. Original sulfides are 50% altered in our NWA 998 section, with iron (oxy) hydroxides at grain boundaries and as complete pseudomorphs. The compositions of unaltered pyrrhotites are homogeneous, close to that of the monoclinic endmember Fe7S8, and are too sulfur‐rich to have been in chemical equilibrium with the late magmatic redox state fixed by the fayalite‐magnetite‐quartz buffer. Therefore, the compositions of the pyrrhotites must have been altered during the later stages of magmatic crystallization, by assimilation of S‐rich regolith and hydrothermal circulation.  相似文献   

5.
Abstract– We have experimentally produced nanophase sulfide compounds and magnetite embedded in Si‐rich amorphous materials by flash‐cooling of a gas stream. Similar assemblages are ubiquitous, and often dominant components of samples of impact‐processed silica aerogel tiles and submicron grains from comet 81P/Wild 2 were retrieved by NASA’s Stardust mission. Although the texture and compositions of nanosulfide compounds have been reproduced experimentally, the mechanisms of formation of these minerals and their relationship with the surrounding amorphous materials have not been established. In this study, we present evidence that both of these materials may not only be produced through cooling of a superheated liquid but they may have also been formed simultaneously by flash‐cooling and subsequent deposition of a gas dominated by Fe‐S‐SiO‐O2. In a dust generator at the Goddard Space Flight Center, samples are produced by direct gas‐phase condensation from gaseous precursors followed by deposition, which effectively isolates the effects of gas‐phase reactions from the effects of melting and condensation. High‐resolution transmission electron microscopy images and energy‐dispersive spectroscopy analysis show that these experiments replicate key features of materials from type B and type C Stardust tracks, including textures, distribution of inclusions, nanophase size, and compositional diversity. We argue that gas‐phase reactions may have played a significant role in the capture environment for nanophase materials. Our results are consistent with a potential progenitor assemblage of micron and submicron‐sized sulfides and submicron silica‐bearing phases, which are commonly observed in chondritic interplanetary dust particles and in the matrices of the most pristine chondritic meteorites.  相似文献   

6.
Abstract— In order to elucidate the formation of low‐H kaersutites in Martian meteorites, the Fe3+/ΣFe ratio of Chassigny kaersutites in magmatic inclusions was directly determined by the synchrotron microXANES analysis. XANES analysis for standard kaersutites with known Fe3+/ΣFe ratios shows a linear relationship between centroid energy positions of XANES pre‐edge spectra and the Fe3+/ΣFe ratio. Based on the linear relationship, the Fe3+/ΣFe ratio of Chassigny kaersutites is estimated to be about 0.05. The low Fe3+/ΣFe ratio clearly suggests that low‐H kaersutites in Chassigny are not likely to be formed by the oxidation‐dehydrogenation reactions. The low‐H content of the Chassigny kaersutites is mainly due to the presence of a Ti oxy‐component.  相似文献   

7.
Pieces of high-silica, natural glass (Libyan Desert Glass), found on the desert surface of western Egypt, have been treated as an enigma for 50 years although it is virtually certain they are similar to tektites in being impact-derived products. New major element analyses of four Libyan Desert Glass specimens agree extremely well with the only other recent analysis and demonstrate that the original bulk analyses reported by Spencer (1939) are in error. The five modern analyses define a very tight chemical range for SiO2 (97.38–98.25 wt %), Al2O3 (1.16–2.26 wt %), total Fe (0.15–0.60 wt % as Fe2O3) and TiO2 (0.13–0.19 wt %). Measurable MgO (0.04–0.20 wt %) was found in one specimen. No other elements are present in greater than trace amounts. Microprobe analyses show that Al, Fe and Ti are all positively correlated with one another and are almost ubiquitously distributed throughout the glass. They must also have been so distributed in the LDG precursor material as mechanical mixing and elemental diffusion in the short-lived melt were limited. In contrast, Mg is sharply restricted in occurrence and correlates only with Fe, strongly suggesting a precursor Mg-Fe oxide or silicate mineral present as rare, discrete grains. Aside from rare accessory minerals, the parent material was a sand or sandstone composed of quartz grains coated with a mixture of kaolinite, hematite and anatase. This conclusion is based solely on the elemental distribution in the glass but is buttressed by the occurrence of both sand and sandstone, in southwestern Egypt, with the requisite mineralogy. However, mineralogic identity need not, in general, translate to a chemical match and it is entirely possible that the specific sand or sandstone facies involved in the glass formation no longer exists after 28 million years. Consequently, it may well be that evidence other than chemical comparisons will be needed to identify the presently unknown parent crater.  相似文献   

8.
Abstract— Among the samples returned from comet 81P/Wild 2 by the Stardust spacecraft is a suite of particles from one impact track (Track 25) that are Ca‐, Al‐rich and FeO‐free. We studied three particles from this track that range in size from 5.3 × 3.2 μ to 15 × 10 μ. Scanning and transmission electron microscopy show that they consist of very fine‐grained (typically from ?0.5 to ?2 μ) Al‐rich, Ti‐bearing and Ti‐free clinopyroxene, Mg‐Al spinel and anorthite, with trace amounts of fine perovskite, FeNi metal and osbornite (TiN) grains. In addition to these phases, the terminal particle, named “Inti”, also contains melilite. All of these phases, with the exception of osbornite, are common in refractory inclusions and are predicted to condense at high temperature from a gas of solar composition. Osbornite, though very rare, has also been found in meteoritic refractory inclusions, and could have formed in a region of the nebula where carbon became enriched relative to oxygen compared to solar composition. Compositions of Ti‐pyroxene in Inti are similar, but not identical, to those of fassaite from Allende inclusions. Electron energy loss spectroscopy shows that Ti‐rich pyroxene in Inti has Ti3+/Ti4+within the range of typical meteoritic fassaite, consistent with formation under reducing conditions comparable to those of a system of solar composition. Inti is 16O‐rich, with δ18O?δ17O?‐40%0, like unaltered phases in refractory inclusions and refractory IDPs. With grain sizes, mineralogy, mineral chemistry, and an oxygen isotopic composition like those of refractory inclusions, we conclude that Inti is a refractory inclusion that formed in the inner solar nebula. Identification of a particle that formed in the inner solar system among the comet samples demonstrates that there was transport of materials from the inner to the outer nebula, probably either in a bipolar outflow or by turbulence.  相似文献   

9.
Abstract— We examined the local iron environment in nine impact glasses from the Cretaceous‐Tertiary (K/T) boundary section at Beloc, Haiti, which formed as the result of impact melting during the Chicxulub impact event. The samples have been analyzed by Fe K‐edge high‐resolution X‐ray absorption near edge structure (XANES) spectroscopy to obtain data on both the Fe oxidation state and the coordination number. The pre‐edge peak of our high‐resolution XANES spectra display noticeable variations indicative of significant changes in the Fe oxidation state spanning a wide range from about 20 to 75 mol% trivalent Fe. All data plot along the same trend, falling between two mixing lines joining a point calculated as the mean of a group of tektites studied so far (consisting of four‐ and five‐coordinated Fe2+) to [4]Fe3+ and [5]Fe3+, respectively. Thus, the XANES spectra can be interpreted as a mixture of [4]Fe2+, [5]Fe2+, [4]Fe3+, and [5]Fe3+. There is no evidence for six‐fold coordinated Fe; however, its presence in small amounts cannot be excluded from XANES data alone. Our observations can be explained by two possible scenarios: either these impact glasses formed under very reducing conditions and, because of their small size, were easily oxidized in air while still molten, or they formed under a variety of different oxygen fugacities resulting in different Fe oxidation states. In the first case, the oxidation state and coordination number would imply similar formation conditions as splash‐form tektites, followed by progressive oxidation.  相似文献   

10.
Abstract— Martian crust endured several large meteoroid impacts subsequent to the demise of an early global magnetic field. Shock pressures associated with these impacts demagnetized parts of the crust, to an extent determined by shock resistance of magnetic materials in the crust. Impacts that form large basins generate pressures in excess of 1 GPa within a few crater radii of their impact sites. Crustal materials near the surface experience significantly reduced impact pressure, which varies with depth and distance from the impact point. We present new demagnetization experiments on magnetite (Fe3O4), hematite (α‐Fe2O3), and titanohematite (Fe2‐xTixO3 where x <0.2). Our measurements show that pressures of ?1 GPa are sufficient to partially demagnetize all of these minerals. The efficiency of demagnetization by impact pressure is proportional to the logarithm of the minerals' magnetic coercivity. The impact pressure magnetic response from exsolved titanohematite samples is consistent with the magnetization decay near Prometheus impact basin and may point to an oxidized igneous rock in Terra Sirenum region at the time of acquisition of magnetic remanence. The remaining magnetic anomalies near large impact basins suggest moderate crustal coercivity. These anomalies point to titanomagnetite as a magnetic carrier and more reduced condition during crustal formation.  相似文献   

11.
Abstract— Films condensed from vapors containing SiO, Fe, or Mg showed an amorphous structure. Infrared (IR) spectra and electron microscopic characterization have been carried out on these films. After the heat treatment of these films in air, IR peaks at approximately 18–23 μm appeared, in addition to peaks attributable to SiO2. These peaks can be attributed to metallic oxides such as FeO, Fe2O3, and MgO. It can be concluded that Fe‐ or Mg‐bearing silicate minerals cannot be produced by the rapid cooling of SiO, Fe, or Mg vapors. Although IR spectra of FeO have been discussed in order to match some spectra obtained with the Infrared Space Observatory, the identification of FeO as the impurity would be very important because the IR spectra of FeO grains are very dependent on the shape and size of the grains. These impurities can also influence the IR spectral feature of SiO2.  相似文献   

12.
Abstract— We report a comprehensive imaging study including confocal microRaman spectroscopy, scanning electron microscopy (SEM), and 3‐D extended focal imaging light microscopy of carbonate globules throughout a depth profile of the Martian meteorite Allan Hills (ALH) 84001 and similar objects in mantle peridotite xenoliths from the Bockfjorden volcanic complex (BVC), Svalbard. Carbonate and iron oxide zoning in ALH 84001 is similar to that seen in BVC globules. Hematite appears to be present in all ALH 84001 carbonate‐bearing assemblages except within a magnesite outer rim found in some globules. Macromolecular carbon (MMC) was found in intimate association with magnetite in both ALH 84001 and BVC carbonates. The MMC synthesis mechanism appears similar to established reactions within the Fe‐C‐O system. By inference to a terrestrial analogue of mantle origin (BVC), these results appear to represent the first measurements of the products of an abiotic MMC synthesis mechanism in Martian samples. Furthermore, the ubiquitous but heterogeneous distribution of hematite throughout carbonate globules in ALH 84001 may be partly responsible for some of the wide range in measured oxygen isotopes reported in previous studies. Using BVC carbonates as a suitable analogue, we postulate that a low temperature hydrothermal model of ALH 84001 globule formation is most likely, although alteration (decarbonation) of a subset of globules possibly occurred during a later impact event.  相似文献   

13.
Abstract— Many of the nanometer‐scale grains from comet 81P/Wild 2 did not survive hypervelocity capture. Instead, they melted and interacted with silica melt derived from the aerogel used by the Stardust mission. Their petrological properties were completely modified, but their bulk chemistry was preserved in the chemical signatures of mostly vesicular Si‐rich glass with its typical Fe‐Ni‐S compound inclusions. Chondritic aggregate IDP L2011A9 that experienced atmospheric pre‐entry thermal modification was selected as an analog to investigate these Wild 2 chemical signatures. The chemical, petrologic, and mineralogical properties of the individual constituents in this aggregate IDP are presented and used to match the chemical signatures of these Wild 2 grains. Mixing of comet material and pure silica, which is used in a diagram that recognizes this mixing behavior, is used to constrain the probable petrologic and minerals that caused the Wild 2 signatures. The Wild 2 nanometer‐scale grain signatures in Si‐rich glass allocations from three different deceleration tracks resembled mixtures of ultrafine‐grained principal components and dense agglomerate‐like material, Mg‐rich silicates (<500 nm) and Fe,Ni‐sulfides (<100 nm), and Si‐rich amorphous material. Dust resembling the mixed matrix of common chondritic aggregate IDPs was present in Jupiter‐family comet Wild 2.  相似文献   

14.
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Cover: Light microscope image of 950 μm‐long Stardust track, C2045,4,178,0,0 (#178), of cometary grains captured in aerogel at 6.1 kms‐1. Hicks et al. discuss the identifi cation of magnetite in this and other Stardust tracks. The magnetite is interpreted to have formed through water‐rock reaction on the comet Wild 2 parent body. Scale bar = 200 μm. Image courtesy of L. Hicks.  相似文献   

15.
We present the analyses results of two bulk Terminal Particles, C2112,7,171,0,0 and C2112,9,171,0,0, derived from the Jupiter‐family comet 81P/Wild 2 returned by the Stardust mission. Each particle embedded in a slab of silica aerogel was pressed in a diamond cell. This preparation, as expected, made it difficult to identify the minerals and organic materials present in these particles. This problem was overcome using a combination of three different analytical techniques, viz. FE‐SEM/EDS, IR, and Raman microspectroscopy that allowed identifying the minerals and small amounts of amorphous carbon present in both particles. TP2 and TP3 were dominated by Ca‐free and low‐Ca, Mg‐rich, Mg,Fe‐olivine. The presence of melilite in both particles is supported by IR microspectroscopy, but is not confirmed by Raman microspectroscopy, possibly because the amounts are too small to be detected. TP2 and TP3 show similar silicate mineral compositions, but Ni‐free and low‐Ni, subsulfur (Fe,Ni)S grains are present in TP2 only. TP2 contains indigenous amorphous carbon hot spots; no indigenous carbon was identified in TP3. These nonchondritic particles probably originated in a differentiated body. This work found an unanticipated carbon contamination following the FE‐SEM/EDS analyses. It is suggested that organic materials in the embedding silica aerogel are irradiated during FE‐SEM/EDS analyses creating a carbon gas that develops a strong fluorescence continuum. The combination of the selected analytical techniques can be used to characterize bulk Wild 2 particles without the need of extraction and removal of the encapsulating aerogel. This approach offers a relatively fast sample preparation procedure, but compressing the samples can cause spurious artifacts, viz. silica contamination. Because of the combination of techniques, we account for these artifacts.  相似文献   

16.
An assemblage with FeNi metal, troilite, Fe‐Mn‐Na phosphate, and Al‐free chromite was identified in the metal‐troilite eutectic nodules in the shock‐produced chondritic melt of the Yanzhuang H6 meteorite. Electron microprobe and Raman spectroscopic analyses show that a few phosphate globules have the composition of Na‐bearing graftonite (Fe,Mn,Na)3(PO4)2, whereas most others correspond to Mn‐bearing galileiite Na(Fe,Mn)4(PO4)3 and a possible new phosphate phase of Na2(Fe,Mn)17(PO4)12 composition. The Yanzhuang meteorite was shocked to a peak pressure of 50 GPa and a peak temperature of approximately 2000 °C. All minerals were melted after pressure release to form a chondritic melt due to very high postshock heat that brought the chondrite material above its liquidus. The volatile elements P and Na released from whitlockite and plagioclase along with elements Cr and Mn released from chromite are concentrated into the shock‐produced Fe‐Ni‐S‐O melt at high temperatures. During cooling, microcrystalline olivine and pyroxene first crystallized from the chondritic melt, metal‐troilite eutectic intergrowths, and silicate melt glass finally solidified at about 950–1000 °C. On the other hand, P, Mn, and Na in the Fe‐Ni‐S‐O melt combined with Fe and crystallized as Fe‐Mn‐Na phosphates within troilite, while Cr combined with Fe and crystallized as Al‐free chromite also within troilite.  相似文献   

17.
Abstract— Fine‐grained, heavily‐hydrated lithic clasts in the metal‐rich (CB) chondrites Queen Alexandra Range (QUE) 94411 and Hammadah al Hamra 237 and CH chondrites, such as Patuxent Range (PAT) 91546 and Allan Hills (ALH) 85085, are mineralogically similar suggesting genetic relationship between these meteorites. These clasts contain no anhydrous silicates and consist of framboidal and platelet magnetite, prismatic sulfides (pentlandite and pyrrhotite), and Fe‐Mn‐Mg‐bearing Ca‐carbonates set in a phyllosilicate‐rich matrix. Two types of phyllosilicates were identified: serpentine, with basal spacing of ?0.73 nm, and saponite, with basal spacings of about 1.1–1.2 nm. Chondrules and FeNi‐metal grains in CB and CH chondrites are believed to have formed at high temperature (>1300 K) by condensation in a solar nebula region that experienced complete vaporization. The absence of aqueous alteration of chondrules and metal grains in CB and CH chondrites indicates that the clasts experienced hydration in an asteroidal setting prior to incorporation into the CH and CB parent bodies. The hydrated clasts were either incorporated during regolith gardening or accreted together with chondrules and FeNi‐metal grains after these high‐temperature components had been transported from their hot formation region to a much colder region of the solar nebula.  相似文献   

18.
The crystalline form of magnetic spherules, retrieved from the Pleistocene and Holocene mineral and organic surface sediments at the Northern foreland of Morasko meteorite-impact site, have been determined. While the magnetite (Fe3O4) composition of the spherules prevails, also hematite, maghemite and plagioclase crystalline inclusions have been observed. The magnetite form varies from amorphous, through fine powder, rough powder, holocrystalline to single crystals, as well as mixed forms, such as powder and holocrystalline, or amorphous and holocrystalline. The relieves on the surface of the spherules reflect the size and shape of the crystallites. The morphology of the spherules, their chemical composition and structure have been characterized and discussed in relation to the Morasko-Meteorite fall, and possible other cosmic, geological or anthropogenic origins.  相似文献   

19.
This thermal annealing experiment at 1000 K for up to 167 h used a physical mixture of vapor phase‐condensed magnesiosilica grains and metallic iron nanograins to test the hypothesis that a mixture of magnesiosilica grains and an Fe‐source would lead to the formation of ferromagnesiosilica grains. This exploratory study found that coagulation and thermal annealing of amorphous magnesiosilica and metallic grains yielded ferromagnesiosilica grains with the Fe/(Fe + Mg) ratios in interplanetary dust particles. Furthermore, decomposition of brucite present in the condensed magnesiosilica grains was the source for water and the cause of different iron oxidation states, and the formation of amorphous Fe3+‐ferrosilica, amorphous Fe3+‐Mg, Fe‐silicates, and magnesioferrite during thermal annealing. Fayalite and ferrosilite that formed from silica/FeO melts reacted with forsterite and enstatite to form Mg, Fe‐silicates. The presence of iron in different oxidation states in extraterrestrial materials almost certainly requires active asteroid‐like parent bodies. If so, the possible presence of trivalent Fe compounds in comet P/Halley suggests that Halley‐type comets are a mixture of preserved presolar and processed solar nebula dust. The results from this thermal annealing experiment further suggest that the Fe‐silicates detected in the impact‐induced ejecta from comet 9P/Temple 1 might be of secondary origin and related to the impact experiment or to processing in a regolith.  相似文献   

20.
Abstract— Oxidation of Fe metal and Gibeon meteorite metal to magnetite via the net reaction 3 Fe (metal) + 4 H2O (gas) = Fe3O4 (magnetite) + 4 H2 (gas) was experimentally studied at ambient atmospheric pressure at 91–442 °C in H2 and H2-He gas mixtures with H2/H2O molar ratios of ～4–41. The magnetite produced was identified by x-ray diffraction. Electron microprobe analyses showed 3.3 wt% NiO and 0.24 wt% CoO (presumably as NiFe2O4 and CoFe2O4) in magnetite formed from Gibeon metal. The NiO and CoO concentrations are higher than expected from equilibrium between metal and oxide under the experimental conditions. Elevated NiO contents in magnetite were also observed by metallurgists during initial stages of oxidation of Fe-Ni alloys. The rate constants for magnetite formation were calculated from the weight gain data using a constant surface area model and the Jander, Ginstling-Brounshtein, and Valensi-Carter models for powder reactions. Magnetite formation followed parabolic (i.e., diffusion-controlled) kinetics. The rate constants and apparent activation energies for Fe metal and Gibeon metal are: These rate constants are significantly smaller than the parabolic rate constants for FeS growth on Fe metal in H2S-H2 gas mixtures containing 1000 or 10 000 ppmv H2S (Lauretta et al., 1996a). The experimental data for Fe and Gibeon metal are used to model the reaction time of Fe alloy grains in the solar nebula as a function of grain size and temperature. The reaction times for 0.1–1 μm radius metal grains are generally within estimated lifetimes of the solar nebula (0.1–10 Ma). However, the calculated reaction times are probably lower limits, and further study of magnetite formation at larger H2/H2O ratios, at lower temperatures and pressures, and as a function of metal alloy composition is needed for further modeling of nebular magnetite formation.  相似文献   

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